Among the long accepted practices to treat a variety of vascular disorders are surgical procedures that involve placement of a tubular graft in a patient's blood vessels. The construction and characteristics of the graft typically are adapted to optimize its use in the specific surgical environment and condition to be treated and, accordingly, a number of different types of grafts are available. Among the most common vascular grafts are those formed from a woven or knitted tubular fabric as well as non-fabric tubes such as expanded polytetrafluoroethylene. Such grafts typically are placed in a patient's vascular system in a highly invasive surgical procedure. In general, the complexity of the surgical procedure required to place the graft will depend on many factors, including the location and surgical accessibility of the portion of the patient's vasculature where the graft is to be placed.
Not all vascular conditions in which it would be desirable to place a graft can be so treated. Among the particularly troublesome medical conditions in which it is desirable to place a graft is that of an abdominal aortic aneurysm, in which a portion of the patient's aorta, the major artery carrying blood from the heart, has developed a weakened wall such that the weakened portion will tend to expand under the influence of the patient's blood pressure. An aortic aneurysm presents a life threatening risk that the aneurysm may burst causing massive internal bleeding. Treatment of the condition typically has involved deeply invasive abdominal surgery in which the patient's abdominal cavity is opened to reach and expose the aortic aneurysm. While maintaining the patient on an independent life support system, the region of the aneurysm is incised lengthwise to enable insertion of the graft into the aorta to span the weakened region and define a structurally strong tubular flow path between the remaining healthy portions of the aorta. The graft, so positioned, then is sutured in place. The graft thus serves as a reinforcing liner for the weakened portion of the aorta. Such surgical procedures have been characterized by a relatively high mortality rate. Typically, patients suffering from the condition are elderly and are less able to survive the rigors of major abdominal surgery. Additionally, there is a substantial degree of risk when the abdominal cavity is opened because the confining pressure of other abdominal organs on the aorta is released. In some cases, the aortic wall in the region of the aneurysm is so weak that upon release of the confining pressure, the aneurysm may burst with resulting immediate massive hemorrhaging.
Significant effort has been directed to the development of less invasive techniques for placement of a graft, such as in the abdominal aortic region, in a manner that presents less risk to the patient. Various devices in the form of a single tubular implant have been described for that purpose. Typically, such single tube devices have a means for anchoring the ends of the graft to healthy vascular tissue at the opposite ends of the aneurysmal region.
In the case of abdominal aortic aneurysms, however, by the time the aneurysm is discovered, it may have advanced to a stage where there is insufficient healthy tissue in the region where the aorta bifurcates into the two iliac arteries for the end of a single tubular graft to be secured. Often the aneurysm has advanced into and involves the iliac arteries as well. Such a condition cannot be treated by a single lumen tubular prosthesis. If an endoluminal prosthesis is to be placed in such anatomy, the prosthesis must have a bifurcated configuration in order to extend fully through the regions of aneurysm in all of the involved arteries and with the device being in secure engagement with healthy tissue to insure that it provides full endoluminal support and will not migrate from its implanted position.
The necessity for the use of a bifurcated graft has been recognized. One approach is described in U.S. Pat. No. 5,489,295 (Piplani) in which a one-piece bifurcated graft is described as being placed in the bifurcated region of the aorta and iliac arteries. The procedure described is complex and requires many difficult manipulations by the physician. Additionally, the unpredictable variations and vascular anatomy of an individual patient can be expected to present blood vessels of shapes and diameters that do not readily match the preformed configuration of a one piece, preformed, bifurcated graft. Consequently, the use of such a graft is likely to result in mismatches between one or more of the three legs of the bifurcated graft with the corresponding aorta and iliac arteries.
Another device is described in PCT patent application PCT/US95/01466 (International Publication No. WO95/21592) in which a bifurcated device is formed in two sections, one of which includes a graft having a main body and one integral leg extension. The main body is intended to be placed in the aorta and the extension in one of iliac arteries. The main body also has an opening adjacent the leg extension that can be aligned with the other iliac artery. The second section of the device includes a tubular graft that can be advanced through the other iliac artery into engagement with the opening in the main body. This arrangement also is awkward to place and presents additional difficulties. Among those difficulties is that where the body and one of the legs is formed in a unitary structure, the device similarly does not lend itself to modular construction in which the individual components can be combined to match the dimensions of the particular patient's anatomy. Also among the difficulties with the device is that it employs stenting elements formed from a shape memory alloy that is not readily visualized under fluoroscopic examination and must be placed relatively quickly, before the stent expands as a consequence of exposure to body temperature.
A third approach, described in PCT application PCT/DK94/00468 (International Publication No. WO95/16406) employs an endovascular device for placement at the region of an arterial bifurcation that includes a three-component system including a main, bag-like body intended for placement in the aorta. The bag-like body has a pair of openings formed at its lower end. Each of the openings is said to receive an end of a tubular prosthesis inserted through each of the iliac arteries. The tubular prostheses are said to attach to the main bag-like body and extend into the iliac arteries. Among the apparent difficulties with this arrangement are that there is no way for the physician to visualize, fluoroscopically, the main bag-like body or the location of the openings that must be accessed in order to insert the leg segments into the openings. Additionally, the apparently flexible structure of the bag-like body would enable the bag to flex under the influence of blood flow, body movement and engagement with the apparatus for delivering and placing the leg segments, all of which would present considerable difficulties in the placement process. Moreover, even if the leg segments could be attached to the main body, the apparent relationship of the leg segments with the main body is such as to present an irregular, turbulent flow path from the main body to the leg extensions.
None of the prior art relating to the placement of a bifurcated tubular endoprosthetic implant within the region of the aortic-iliac arteries has suggested a modular construction by which the components can be accurately positioned, assembled and, if necessary, is recaptured within the delivery device so that the module can be redeployed in its proper position or removed in its entirety. Additionally, none embodies an arrangement in which the leg portions of the stent have a high degree of flexibility in order to conform easily to the configuration of the particular patient's vascular anatomy.
It would be desirable to provide a modular implant that can be constructed in situ with modules that can be accurately placed and connected easily with individual modules selected to conform closely to the vascular anatomy of the individual patient. It is among the general objects of the invention to provide such a modular implant construction.